Elastomers are typically cured by first being compounded with a curing agent, processing agents, and other additives such as reactive coagents and fillers to form a masterbatch, and then being heated to temperatures at which curing occurs. Suitable curing agents for many elastomers include organic peroxides which produce free radicals in the elastomer masterbatch. The free radicals initiate cross-linking of the elastomer polymer chains to cure the elastomer. Processing aids are added to the masterbatch to reduce the viscosity of the masterbatch during compounding so that the curing agent and other additives can be thoroughly and rapidly dispersed in the elastomer.
Even with the addition of processing aids, some elastomers, such as nitrile rubber and EPDM, have a relatively high viscosity and must be compounded at relatively high temperatures so that the viscosity is low enough for thorough and rapid dispersion of the curing agent and other additives in the elastomer. High viscosity elastomers such as nitrile rubber are typically compounded at a temperature within the range from about 38.degree. to about 130.degree. C. and then cured at a temperature within the range from about 150.degree. to about 205.degree. C.
To form useful products, elastomers are typically formed into a useful shape, such as by injection molding, after compounding but before curing of the elastomer. High viscosity elastomers such as nitrile rubber must also be at a high temperature during formation into a useful shape.
Curing increases the viscosity of elastomer compositions. Thus, it is desirable to delay curing during compounding of an elastomer masterbatch to allow time for thorough dispersion of curing agents and other additives in the masterbatch. Likewise, it is desirable to delay curing during formation of the elastomer into a useful shape so that the shape can be completely formed with a reasonable amount of forming energy.
After compounding and formation are complete, it is desirable, in applications where relatively hard, strong rubber is desired, to cure the elastomer masterbatch rapidly and develop a high level of cross-linking and modulus in the rubber. Reactive coagents such as unsaturated carboxylic metal salts promote rapid curing of elastomers when organic peroxides are used as the curing agent and also enhance the cross-linking and modulus in the final product. Elastomers cured with organic peroxide curing agents and unsaturated carboxylic metal salt can have physical properties superior to elastomers cured with sulfur, but tend to precure (scorch) during compounding or formation. This has been a particular problem when using zinc diacrylate as the reactive coagent in the curing of elastomers such as nitrile rubber and EPDM, which require high compounding and formation temperatures. The unsaturated carboxylic metal salt tends to form free radicals at high compounding and formation temperatures.
Precure increases the viscosity of the masterbatch and can make compounding and formation more difficult. This can result in shriveling and lumpiness in the cured product. In extreme cases, precure can cause "freezing" of the masterbatch in the compounding, forming, and curing, apparatus. To solve this problem, precure inhibitors (also known as prevulcanization inhibitors or scorch retarders) have been developed, examples of which are disclosed in European Patent 319,320A and U.S. Pat. Nos. 4,843,114; 4,857,571; 4,918,144; 4,983,678; and 4,990,570. There remains, however, the need for a precure inhibitor that is relatively non-hazardous, does not diminish the physical properties of the elastomer, and is economical when used in elastomer compositions comprising an organic peroxide as the curing agent and an unsaturated carboxylic metal salt as a reactive coagent.